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Ankylosing Spondylitis Research Paper

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ANKYLOSING SPONDYLITIS
Darren Bergey M.D., Robert S. Pashman. M.D.
J. Patrick Johnson M.D.
INTRODUCTION
Ankylosing spondylitis is a seronegative spondyloarthropathy which primarily affects the axial
skeleton, including ligaments and articulations of the pelvis and spinal column. It is estimated to occur
in 0.02 % of the population. Although once believed to affect men predominantly, recent evidence
suggests women are affected equally but experience milder symptoms. The HLA-B27 antigen is
positive in 80-90 percent of patients with ankylosing spondylitis compared with 8 percent of the
general population of American Caucasians (28). This strongly suggests that HLA-B27 antigen is
important in the pathogenesis of ankylosing spondylitis. Its precise role remains unclear, however, it is
generally understood that an inflammatory response is incited by environmental or infectious agents
and hosts are rendered susceptible by HLA-B27 or related antigens (37).
Similar to rheumatoid arthritis, the pathophysiology of ankylosing spondylitis remains unclear. The
basic pathologic process is an inflammatory focus, predominantly lymphocytic, that targets both
articular joints as well as the insertion of ligaments, tendons and capsules to bone (entheses). Reactive
bone formation at these entheses, termed enthesopathy, ultimately results in progressive ankylosis of
the axial skeleton, typically involving the sacroiliac, apophyseal, and costovertebral joints. The course
of the disease includes progressive enchondral ossification of cartilage, resulting in the characteristic
joint stiffness and ankylosis. Symptoms usually begin in at the sacroiliac joints and progress
proximally in the spine.
The “Romanus lesion” is an erosion of the anterior and lateral border of the vertebral endplate at the
site of vascular attachment of the annulus fibrosus. This lesion represents a focal area of spondylitis,
ultimately resulting in syndesmophyte formation and ossification of the annulus fibrosus. These
osseous changes result in the classic “bamboo spine” appearance radiographically, which is the
hallmark of anklylosing spondylitis.
CLINICAL PRESENTATION
Ankylosing spondylitis typically presents in healthy adults during the second or third decades.
Sacroiliitis or low back pain is typically the initial manifestation of the disease. Pain may be unilateral
or bilateral and may include radicular symptoms extending into the buttocks or thigh. This radicular
pain seldom extends below the knee. Symptoms are usually worse in the morning and improve with
activity. This clinical feature distinguishes ankylosing spondylitis from mechanical low back pain,
which generally worsens with activity and improves with rest. Night pain relieved by activity is not an
uncommon feature of ankylosing spondylitis (28). In patients with an uncontrolled inflammatory phase of the disease, the lumbar, thoracic, and cervical
spine become progressively ankylosed and kyphotic. This usually progresses in a caudal to cranial
direction. According to Simkin and colleagues, kyphosis is produced when the patient assumes a
“flexed posture in an attempt to unload the facets, thereby reducing joint pressure and alleviating pain
(51).” Compensatory flexion contractures of the hips and knees may develop as the patient attempts to
maintain an erect posture. Following the inflammatory phase, the patient is typically stiff and kyphotic
but relatively pain free. Significant spinal deformity and functional disability may be the end result.
Carette and associates studied 150 war veterans for a mean of 38 years and found that, despite severe
limitations in spinal motion, 50 per cent of the patients functioned well (13). Those with more severe
deformities, however, may be unable to stand upright, lose horizontal gaze, and develop the so-called
chin-on-chest deformity.
Spinal fracture and spondylodiscitis are clinical manifestations of ankylosing spondylitis which are of
specific interest to spine surgeons. Ossification of the disc space occurs centripetally through the
annulus fibrosus, and only rarely is the center of the disc involved. This incomplete ossification
combined with stress concentration from loss of polysegmental spinal motion and secondary
osteopenia predispose patients to spinal fracture and nonunion (spondylodiscitis) (26).
Spondylodiscitis presents as focal pain with coexisting erosive sclerotic changes in adjacent vertebral
bodies. It is uncertain whether this is a primary inflammatory process or the result of trauma.
Radiographically, the appearance of spondylodiscitis, pseudarthrosis and discitis are very similar.
Neurologic decline in the patient with ankylosing spondylitis is uncommon, exclusive of fractures.
However, neurologic injury can be a significant complication of spinal fracture and the diagnosis
should not be missed. Severe spinal deformities, together with spondylodiscitis and acute fracture
provide the most common indication for spine surgery in the patient with ankylosing spondylitis.

SURGICAL MANAGEMENT
General Principles
As ankylosing spondylitis may lead to severe flexion deformities of the spine the goal in treatment of
these patients is early recognition and adequate medical therapy in an attempt to control the disease
progress and prevent associated deformities. However, patients may still become grossly deformed
and functionally disabled. Spinal osteotomy may be indicated to correct the deformity and achieve
upright posture.
Two considerations determine the technique and location of the osteotomy: the region of the spinal
deformity that maximally influences sagittal alignment and a surgical procedure that minimizes the
surgical risk. It is important reemphasize that the overall spinal balance as well as the hips must be
evaluated to delineate the primary site of deformity. In some patients more than one of these sites may
contribute to the deformity. The lumbar spine is the most common site of deformity followed by the
thoracic and cervical regions. Accurate measurement of the deformity is required for surgical planning
and Simmons (52) advocates the chin-brow to vertical angle as the most effective and reproducible measurement of deformity (Fig. 1a,b).
Deformities isolated to the lumbar spine are corrected by a lumbar osteotomy procedure. The
osteotomy is preferred below the level of the conus medullaris and is usually performed at L3 in order
to avoid acute angular correction at the cord level (52). Most lumbar-thoracic kyphotic deformities can
also be addressed through a single lumbar osteotomy. The correction should be planned so that the
plumb line from C7 falls within the body of S1. Even in cases in which the thoracic kyphosis is greater
than normal, a compensatory lumbar osteotomy may correct sagittal plane malalignment and allow the
patient to have forward gaze with the hips and knees fully extended. In cases of severe thoracic
kyphosis, where the lumbar and cervical lordosis have been at least partially maintained, thoracic
osteotomy by a combined anterior and posterior approach may be indicated. When the primary
deformity is at the cervical-thoracic junction, resulting in a ‘chin-on-chest’ deformity, an extension
osteotomy of the cervical spine is indicated. The C7-T1 junction is the preferred location as it places
the osteotomy below the entrance of the vertebral arteries into the transverse processes at C6 and uses
the relatively large spinal canal-to-cord area ratio to safely obtain correction.
The influence of severe hip flexion contractures with or without associated hip joint disease is critical
in the preoperative assessment. Soft tissue releases about the hips or, more commonly, total hip joint
arthroplasty may be sufficient in itself to allow the patient to stand reasonably upright and see straight
ahead, irrespective of the spinal deformity (5). These should be performed prior to any surgical
correction of spinal deformity.
Preoperatively, patients should be screened for cardiac and pulmonary abnormalities that can be
associated extra-articular manifestations of ankylosing spondylitis. Although pulmonary function
abnormalities secondary to decreased thoracic expansion have not carried anesthetic risk for most
patients (20), 10 per cent will have cardiac pathology, generally either aortic stenosis or conduction
abnormalities.
Although local anesthesia has been reported in the treatment of these spinal deformities (32, 57),
general anesthesia is preferred. Intubation is facilitated by the use of fiber optic guidance in cases
where cervicothoracic kyphosis complicates easy passage of the endotracheal tube. After the patient
has been anesthetized and intubated, the operating table must be modified according to the patient’s
spinal deformity. The table is flexed into a position where the apex of the table is under the primary
spinal deformity. Bolsters are used to free the abdomen and protect bony prominences and peripheral
nerves in the extremities.
Lumbar Osteotomy
Smith-Petersen and colleagues’ first proposed lumbar osteotomy for the correction of spinal deformity
caused by ankylosing spondylitis in 1945 (55). They performed a V-shaped wedge resection osteotomy
at the L3-L4 level. The spinous processes were removed at the appropriate angle as were the L3-L4
lamina. The osteotomy was extended laterally to include the bilateral facet joints. The lamina and
pedicles were undercut to prevent impingement of the dura or nerve roots upon closure of the
osteotomy site. This osteotomy wedge was then closed and the deformity corrected via forceful
manipulation through hyperextension. This maneuver caused fracture of the anterior and middle columns allowing the osteotomy to close. Osteotomes may be used to complete the fracture if the
manual maneuver is unsuccessful. Local bone grafts were placed across the osteotomy sites, and the
patient was immobilized in a postoperative cast for 2 months followed by a back brace for one year.
Six patients were reported and detailed results are not described.
Simmons used the Smith-Petersen osteotomy and popularized the use of local anesthetic for both
lumbar and cervical osteotomies, arguing that two thirds of the 8 to 10 percent mortality and 30 percent
neurologic complications documented in previous studies were related to the use of general anesthesia
(52). In his series of 90 patients he was able to show that correction can be reliably achieved through
this posterior osteotomy without a secondary anterior approach, paralleling the experience of other
authors (25,26,37,54,63). His series reported a 40 to 104 degree correction with an average of 56
degrees (Fig. 2). The chin-brow to vertical angle improved from an average of 60 degrees
preoperatively to 5 degrees postoperatively.
The most common complication associated with this procedure is neurologic compression. In
Simmons series seven (8%) patients developed nerve root or cauda equine symptoms postoperatively
(52). Such complications can be minimized by adequate decompression and undercutting of the
lamina prior to closure of the osteotomy site and rigid stabilization. If these complications occur,
prompt reexploration and decompression should be performed.
This osteotomy advocated by Smith-Peterson and Simmons uses the middle column as the fulcrum for
closure of the posterior osteotomy and has the inherent risk of placing the spinal cord on stretch.
Thomasen reported a spinal column shortening osteotomy via a posterior approach utilizing a
decancellization procedure (56) (FIG. 3). The decancellization procedure, also known as an ‘eggshell
osteotomy’ or ‘pedicle subtraction osteotomy’, is performed by removing a wedge of the posterior
elements of L3 as well as bilateral pedicles. This is followed by resection of the posterior vertebral
cortex as well as the cancellous bone of the vertebral body. The anterior cortex of the vertebral body is
left intact and is the fulcrum for closure, effectively shortening the spinal canal and achieving angular
correction. Moreover, removal of the pedicle creates a “super-foramen,” which transmits the nerve
roots from the adjacent segments and decreases the chance for root compression. Generous
undercutting/decompression of the supra and sub adjacent laminar edges are performed to ensure
adequate space for the redundant dura that may be produced during closure of the osteotomy.
Segmental spinal fixation utilizing multiple pedicle screws and/or hook constructs are used to allow for
immediate patient mobilization. Carefully, the table is extended, closing the osteotomy. If necessary,
closure can be augmented by pressure on the patient’s shoulders or legs and by compression between
the pedicle screws once the rods are placed. A wake-up test is routinely performed to assess neurologic
function. Finally, a local bone graft is applied and augmented with iliac crest autograft or banked bone,
as needed. Thomasen reported 12 to 50 degrees of correction in 11 patients, with 5 of the 11 having a
correction of less than 35 degrees (56). He concluded that this small degree of correction was all that
was required to obtain an erect posture. This technique is procedure of choice at this time.

Thoracic Osteotomy
Thoracic osteotomies are rarely required in patients with ankylosing spondylitis. As stated previously,
if the thoracic kyphosis is mild or moderate and associated with a flat or kyphotic lumbar spine the deformity can be addressed with a lumbar spine osteotomy. The rare patient will have severe thoracic
kyphosis with minimal loss of lumbar or cervical lordosis. This is the patient in whom a thoracic
osteotomy may be indicated.
Smith-Petersen pointed out in 1945 that single-stage posterior thoracic osteotomy correction is
compromised by stiffness of the costovertebral joints. We favor a two-stage procedure that consists of
a first-stage transthoracic approach creating osteotomies through the ossified thoracic disc spaces.
Anterior interbody fusion is performed with autogenous cancellous bone graft. This is followed at the
same sitting or one week later by posterior, multiple level Smith-Peterson osteotomies with segmental
instrumentation. Dural adhesions to the lamina that formed during the inflammatory phase of the
disease can be encountered during posterior osteotomy and likewise may make passage of sublaminar
wires used in the Luque technique more difficult. We therefore prefer to stabilize the osteotomy with
hook-rod compression instrumentation or thoracic pedicle screws. The approach is similar to that
used for severe juvenile kyphosis (7,9).
Cervical Osteotomy
Cervical osteotomy may occasionally be indicated when the primary deformity is isolated at the
cervical-thoracic junction. In 1953, Mason and associates reported successful correction of flexion
deformity of the cervical-thoracic spine in a patient with ankylosing spondylitis (40). They performed
the osteotomy distal to C7 in order to avoid damage to the vertebral arteries. In 1958, Urist reported a
successful osteotomy at the cervical-thoracic junction in a patient awake under local anesthesia (59).
Simmons has reported experience in 95 patients, consisting of a wide laminectomy from C6 to T1 with
osteotomy at the C7-T1space (53). The entire posterior arch of C7 is resected, as is the inferior half of
C6 and the upper half of T1. The lamina are undercut and complete foraminotomies are performed to
prevent impingement of the C8 nerve root. The amount of bone to be resected is based on the
preoperative chin-brow to vertical angle. This angle is transferred to the lateral radiograph, with the
apex of the angle at the posterior edge of the C7-T1 disc space. Following bony decompression
Simmons extended the neck and “cracked” the anterior column (FIG 4). An osteotome may be used to
perform an anterior osteotomy in a more controlled fashion. Simmons performed the procedure under
local anesthesia with halo control and then fixed the halo to a body cast which was worn for 4 months.
There were no mortalities and C8 weakness was the primary morbidity occurring in 18 patients, 5
being permanent deficits.
We believe the preferred technique for cervical osteotomy to be that described by Urist and Simmons
(59,53). We prefer general anesthetic with controlled halo correction, followed by either an
intraoperative wake-up test or spinal cord monitoring. Lateral mass screws are used in the cervical
spine with thoracic pedicle screws placed in the upper thoracic spine for interal fixation. Halo and vest
supplementation may or may not be utilized.
Complications Postoperative ileus is common in these patients. Nasogastric drainage is essential. Although aortic
rupture has been reported (39, 50, 60) the case in question occurred after closed forceful osteoclasis of
severe kyphosis in a patient who had previously been treated with radiation therapy for ankylosing
spondylitis. We believe that the fear and likelihood of this complication has been greatly overstated.
It has been stated in review of several series (43-57) that mortality has varied from 8 to 10 percent,
and neurologic complications have occurred in up to 30 per cent of patients. However, these quotes
may be misleading. In our analysis of the 14 largest series consisting of five or more cases reported,
(17-21, 36-39 43, 53, 54, 56, 57) a total of 427 cases, we find a 4 per cent incidence of neurologic
complications and a 5 percent mortality rate. However, and perhaps even more importantly, it appears
that in eight of these reports, consisting of 74 patients, there were no neurologic deficits. In nine of
these series with a total of 85 patients,(17-21,39, 54, 56) no deaths were reported. In the single
largest study, 177 patients reported by Hehne and associates, there was 2.3 per cent mortality and 2.3
per cent irreversible root lesions (24). Based on our review of the published data and our own
experience, we believe that neurologic complications and mortality can be greatly lessened if not
prevented altogether by careful attention to four critical factors: (1) avoiding compression of
neurologic tissue, (2) monitoring neurologic function during the osteotomy (by wake-up test), (3)
using internal fixation, and (4) avoiding translational displacement at the osteotomy site.